Use box layout for css setting

packages/apputils/src/mainareawidget.ts

@@ -39,7 +39,6 @@ class MainAreaWidget<T extends Widget = Widget> extends Widget {
     this.addClass('jp-MainAreaWidget');
     this.id = uuid();
     let content = this.content = options.content;
-    content.addClass('jp-MainAreaWidget-content');
     if (!content.id) {
       content.id = uuid();
     }

packages/apputils/style/mainareawidget.css

@@ -4,17 +4,7 @@
 | Distributed under the terms of the Modified BSD License.
 |----------------------------------------------------------------------------*/
 
- .jp-MainAreaWidget {
-   display: block;
-   height: 100%;
- }
-
-
 .jp-MainAreaWidget > :focus {
   outline: none;
 }
 
-
-.jp-MainAreaWidget-content {
-    height: calc(100% - var(--jp-private-toolbar-height));
-}